It is often required that electronically powered equipment be protected by a monitoring system that determines the quality of two or more alternating current power sources. The source quality determination is used to decide the suitability of powering a load with a given source. Often the determination of source quality is used to switch from an unsuitable source to a better source. If, for example, one source is above or below pre-set limits, the abnormal voltage condition could damage the load. Also, momentary spikes or sages in voltage can damage the load if the magnitude deviations are of a sufficient magnitude and duration.
Accurate assessment of source quality requires that voltage abnormalities of small magnitude or short duration not cause a source to be categorized as unsuitable. Nuisance switching of loads from one source to another is inefficient.
Prior art source quality assessment techniques typically involve a step of rectifying a three phase voltage, adding the three rectified signals together and smoothing the resulting waveform with a time constant filter. Variations on this basic technique have been used and include rectifying and adding multiple phase shifted copies of the three phase voltages together to improve response time.
U.S. Pat. No. 5,229,651 to Baxter, Jr. et al. which issued Jul. 20, 1993 discusses alternating current monitoring of line signals for use with an uninterruptible power supply. Line signal faults are detected by comparing a presently received cycle of the AC power line signal with a reference waveform that is formed from a composite of waveforms from prior cycles of the AC line signal. An excessive deviation of the current waveform from the reference waveform results in a fault being declared and causes the uninterruptible power supply to provide backup power.